1. Field of the Invention:
This invention relates to a phosphate fluorescent substance. More particularly, this invention relates to a phosphate fluorescent substance having high brightness and high brightness maintenance ratio and specific properties.
2. Description of the Prior Art:
A phosphate fluorescent substance whose parent material is orthophosphate of divalent metal including zinc and whose main activator is manganese (referred to as phosphate fluorescent substance hereinafter) emits red light of high brightness under electron beam excitation and is commonly used for cathode-ray tubes (CRTs). As this phosphate fluorescent substance has the property of prolonged 10% afterglow time (the time required for the brightness to degrade to a value of 10% after excitation ceases), it is widely used for CRTs, especially for CRTs for displays which adopt a scanning speed slower than the CRTs of color TVs, such as computer terminal display devices and the indicating devices of a flight control system.
This kind of phosphate fluorescent substance is basically composed of a phosphate fluorescent substance which is represented as Zn.sub.3 (PO.sub.4).sub.2 : Mn, and part of the main material is replaced or other co-activators or additives are contained in its composition, as is shown in the prior art, such as Japanese Patent Publication No. 18471/1978, or Japanese Patent Public Disclosure Nos. 121258/1981, 136873/1981, 87487/1982 and 40380/1983 by the applicant of this invention.
However, these phosphate fluorescent substance compositions suffer from pronounced degeneration of the brightness compared with other general phosphate fluorescent substance compositions under electron beam excitation over long periods and, additionally, their initial brightness is not sufficient. For these reasons, when they were used in CRTs for displays and the like, color drift is disadvantageously caused by degeneration of the brightness or the current balance is disturbed, because the service current of such CRTs has to be increased above that of electron guns for exciting other luminescent color fluorescent substances because of the insufficient initial brightness.
Further, a phosphate fluorescent substance has excellent solubility in water and is liable to become hydrate by hydrolysis, and therefore it has been observed that a phosphate fluorescent substance may stop emitting from its surface part in the correct sequential order.
In order to avoid this phenomenon various devices have been adopted in producing a fluorescent substance so as not to prolong the wet state during the treatment step after firing and during the step of producing fluorescent films for cathode-ray tubes, but these have required special treatment means which are disadvantageous not only technically but also economically.